In optical imaging systems, imaging performance is of great importance. Factors that influence the imaging performance of the optical system include for example, inherent factors such as lens aberration, diffraction, and refractive index, and external factors such as environmental conditions. Especially chromatic aberration due to a differing refractive index of the lens for each wavelength is a factor that can harm the imaging performance.
Conventional techniques try to reduce the chromatic aberration by combining several lenses with differing Abbe number. Alternatively, it is also known in the art to use anomalous dispersion glass as an achromatic glass system.
As another technique for reducing the chromatic aberration, it has been proposed to add an independent diffraction element to the optical imaging system. Publication of Unexamined Patent Application (Tokukai) No. Hei 8-43767 for example discloses a technique to correct the chromatic aberration by combining a plate diffraction element with a refractive lens. Furthermore, Tokukai No. Hei 6-242373 suggests an objective lens for an optical disk employing a diffractive element on the surface of the lens.
However, in the conventional techniques described above, it is difficult to lower the costs from the viewpoint of lens production and assembly, because a certain number of lenses is needed. In other words, those techniques all result in a rising number of parts, because it is necessary to employ an element not directly involved in the convergent effect to correct the chromatic aberration. Moreover, the technique of employing an independent diffraction element on the lens surface cannot be used for an optical imaging system, because the wavelength range of such a technique is narrow and its angle of view is small.